WO 2009/077727 describes an optical sensor for monitoring environmental parameters such as temperature and/or pressure in extreme environments. For example, the sensor is adapted for use at the high temperatures inside gas turbines. The sensor comprises a sensor element which includes an enclosed cavity arranged as a Fabry-Perot cavity, and an optical fibre. Conventional silica optical fibres cannot withstand temperatures above 600° C. whereas the temperature inside gas turbines exceeds this and may be up to 1000° C. Indeed conventional materials for the sensor element, such as silicon, are not able to withstand such extreme temperatures.
The device described in WO 2009/077727 addresses these problems by providing a sensor element of sapphire and spacing the optical fibre away from the sensor element such that it is not in the extreme temperature environment. This allows a conventional optical fibre to be used. The device is shown in FIG. 1 and comprises a sensor element 10 formed of dielectric material such as sapphire. The sensor element 10 includes a Fabry-Perot cavity enclosed in the element. The sensor element 10 is bonded to spacer 20 which may be a sapphire tube or rod. A waveguide such as an optical fibre 30 delivers light to the sensor for illuminating the sensor element.
The sensor element 10, spacer 20, and optical fibre 30 are mounted to a housing 40. The housing 40 includes a circular socket 45. The optical fibre is mounted in fibre mount 50 which includes a ball 55. The ball 55 fits into the circular socket 45 of housing 40. A lens 60 is mounted close to, or on the end of the optical fibre, inside the fibre mount. The lens 60 provides a collimated beam which is directed to the sensor element 10. A collimated beam is used because the beam does not significantly diverge over the length of the spacer 20. A cap 70 fits over the spacer 20 and sensor element 10 to protect it from mechanical damage. The cap 70 may include a hole 75 through which gas of the surrounding environment may flow. The hole 75 reduces thermal lag between the environment and the sensor element 20. A protective boot 80 is fitted at the other end of the device to protect the optical fibre.
The sensor element 10 is a dielectric body, sometimes known as a “pill”. The pill is shown in detail in FIG. 2 and consists of a disc 12 of sapphire with a small circular recess 14. The disc 12 is bonded to another piece of sapphire 16 which forms a back plate. The disc 12 is thinned to produce a flexible membrane 13 which may flex in response to external pressure. The sapphire surfaces form one or more Fabry-Perot cavities that can be interrogated optically using the collimated light from fibre 30. For example, the distance “A” between the flexible membrane 13 and the back plate 16 provides a cavity which is responsive to changes in pressure. The back plate 16 has front and back surfaces which form a cavity. Thus, the thickness “B” will be responsive to changes in temperature. The cavity “A” will also be responsive to changes in temperature but by interrogating more than one cavity simultaneously the pressure measurement may be corrected for temperature changes. As well as interrogating the cavity “B” of the back plate 16, the thickness “C” of the membrane 13 may be interrogated to provide temperature measurements in front of and behind the pressure cavity to allow accurate interpolation of the temperature of the pressure cavity. This allows the change in cavity size “A” due to pressure to be decoupled from the change in “A” due to temperature.
In an alternative arrangement requiring only pressure measurement, and in which errors due to temperature variation can be tolerated, or in the case of dynamic pressure measurement where only the change is required, the external faces of the pill 10 are angled such that only the surfaces of hollow cavity 14 form a Fabry-Perot cavity. Furthermore, the pill 10 shown in FIG. 2 is interrogated using interferometric techniques also described in WO 2009/077727. These techniques may use phase modulated light. If only the pressure is required then simpler intensity measurements are sufficient. This may be advantageous because the extra cavities may interact with the interconnect to the fibre and can cause higher than expected temperature and vibration sensitivity.
As mentioned above, the prior art device allows the sensor element, or pill, 10 to be held and subjected to extremes of temperature. Alignment of the optical system must be maintained over the working temperature range such that light is stably coupled from fibre 30 to sensor element 10 and back without significant signal variation. The spacer 20 allows the optical fibre 30, which is a conventional silica optical fibre, to be in a region cooler than the sensor element 10.
The spacer 20 is a sapphire rod or tube which fits into housing 40. The housing 40 is made of Kovar® and includes a tube in which the spacer rod or tube 20 is a compression fit. The fibre 30 and lens 60 are mounted in fibre mount 50. The lens 60, which collimates the light leaving the fibre 30, is a compression fit in the fibre mount 50. The ball 55 of the fibre mount is fitted into the socket 45 of housing 40. During assembly the fibre mount 50 is held in a gimbal providing two axes of rotation. The position is adjusted until the fibre and lens are aligned to provide maximum optical throughput to the sensor element 10. In practice this is monitored by sending light along the fibre and adjusting the position of the fibre mount until maximum back reflection from the sensor element is detected in the fibre. A fibre coupler is used to split the back reflected light for monitoring. When the optimum position of the fibre mount is obtained, the ball is fixed to the socket with welds, such as laser welds.
There are problems with the device of WO 2009/077727. At the back end the process of aligning the fibre mount to the housing is time consuming and the alignment may drift during burn-in. The compression fit of the collimating lens 60 in ball 55 causes problems due to holding the silica lens which is a hard material in the Kovar over an extended temperature range without causing drift in the optical alignment. The compression fit also requires high tolerances and assembly is labour intensive.
Furthermore, at the front end the spacer 20 is a compression fit in Kovar tube of housing 40, but this fit does not give a rugged seal that lasts for the life of the sensor. The alignment of the spacer 20, and therefore also the sensor element 10, to the collimated beam drifts due to flexure in the housing 40 and drift of the spacer in the tube. Flexure in the housing is particularly problematic because a small angular deviation in the position of the lens may produce a large lateral displacement of the beam at the sensor element thereby causing a large lateral displacement of the back reflected beam reducing coupling to the fibre. This reduction in signal may reduce the dynamic range of the sensor.
It is an object of the present invention to overcome these problems.